A light emitting diode (“LED”) is a semiconductor light source. LEDs provide numerous advantages over other light sources such as incandescent lights. Amongst other advantages, LEDs typically have longer lifetimes, greater reliability, faster switching characteristics and lower energy consumption. Recent advances have produced LEDs with luminous intensities (lumen per Watt) that are comparable with or surpass incandescent lights.
LED produce light by the spontaneous recombination of electron and hole pairs when the LED is forward biased above the threshold voltage in an electronic circuit. The wavelength of the light produced depends upon the band gap between the materials used in the p-n junction that form the LED. The wavelength of the light produced by an LED is typically in the infra red, visible or UV ranges. Detailed information on LEDs is found in “Light emitting diodes” by E. Fred Schubert, Cambridge University Press, which is hereby incorporated in its entirety by reference. Detailed information on semiconductor optics is found in “Semiconductor optics” by Claus F Klingshirn, Springer press, which is hereby incorporated in its entirety by reference.
For ease of manufacturing, the most common form of LED is a micron-sized planar square LED die disposed on a substrate. The semiconductor comprising the die is usually silicon, and the substrate may be a metal such as aluminum, which also functions as a heat sink. The LED die is electrically connected to circuitry on the substrate by fine metal wires. The LED die itself may be surface mounted on the substrate, or within a cavity on the substrate.
Various challenges exist in producing a LED with a high luminous output that is suitable for use as a light source in human environments. The first is maximizing light extraction from the planar LED die itself. As semiconductor materials have a high refractive index, a large quantity of the light produced undergoes total internal reflection (TIR) at the semiconductor-air interface. It is known in the art to reduce the amount of light that undergoes TIR by reducing the difference between the refractive indices at the semiconductor surface. As the semiconductor refractive index is a material characteristic, this is achieved by encapsulating the LED with an encapsulating material having a higher refractive index. Historically an epoxy material was used, and more recently silicone due to its comparatively higher transparency, color stability and thermal performance. Silicone, however, is relatively harder to dispense.
Planar LEDs have a Lambertian emission pattern, as shown in FIG. 1. In other words, the radiant intensity is directly proportional to the cosine of the angle between the observer's position and the surface normal. Consequently, the apparent radiance of the emitted light to an observer appears the same from all positions. The encapsulating material used to reduce TIR can also act as a lens and narrow the light beam emitted. Various shapes of encapsulating material have been used and it is known in the art to use a hemispherical shaped encapsulant material to maximize the light extraction efficiency from a planar LED die.
Some of the light that does escape is absorbed or reflected by the surrounding substrate material, especially when the LED die is located within a cavity.
A single unencapsulated LED produces monochromatic light. Due to the interest in using LEDs as ambient lighting sources, in recent years research has focused on producing LED packages that emit light of different colors to that emitted by the LED die. There has been considerable interest in producing white light. The most popular way of producing white light from a single LED is by disposing a wavelength converting material, such as a yellow phosphor, on the visible (emitting) side of a blue-light emitting LED die. A layer of wavelength converting material applied on the LED die will absorb some of the emitted photons, and down-convert them into visible wavelength light, resulting in a dichromatic light source of blue and yellow wavelength light. If the yellow and blue light is produced in the correct proportions it is perceived by the human eye as a white color.
It is known in the art to add a wavelength converting material to the encapsulating layer surrounding the LED die. Application of the encapsulating material may be by different methods. They can be divided into two types: those that use molding or pre-molding to fix directly to the substrate, and those that create and then fill a dam that encircles the LED die. The latter is usually referred to as dispensing as the encapsulating material is provided in liquid form followed by curing.
Factors affecting the color quality of the white color light are the quantity and distribution of the phosphor over the LED die. These determine the proportion of yellow wavelength light produced. When the phosphor is dispersed within the encapsulating material, process control issues can result in uneven phosphor distribution surrounding the LED. As a result, the light from the packaged LED may have a yellow “ring” away from the central beam axis. This also arises from the Lambertian emission profile of the LED and/or phosphor particle settling: there are too few phosphor particles on the center of the LED die to emit enough yellow light relative to the amount of blue light emitted, whilst towards the LED die sides there are too many phosphor particles relative to the amount of blue light emitted, and hence over stimulated to emit yellow light.
Thus there is a need in the art for packaged LEDs having uniform color emission properties.